Abstract
The behavior of Dall's porpoise was studied in the western Johnstone Strait, British Columbia, during the summer of 1984. Observations and theodolite trackings were made from a cliff-top observation point on West Cracroft Island. Dall's porpoise was the second most common marine mammal in the study area, being seen on 63% of the days, and it was reported to be a year-round resident of the Strait. Three types of surfacing behavior were observed, with slow rolling occurring in 97% of the sightings. Swimming speed of slow rolling porpoises was 5.1 ± 0.83 (SD) km/h (n = 18). Dives during travel averaged 35.9 ± 26.52 (SD) s (n = 10). Deep vertical dives lasted 2 to 4 min for adults and up to 2 min for calves, and were presumed to be associated with feeding. Westward movement predominated. Dall's porpoises usually showed no obvious reaction to resident killer whales, but apparently swimming speed increases when killer whales are within 2 km. Group sizes ranged from one to over five animals (mean size = 2.6 ± 1.05 (SD), n = 68). Groups that included calves tended to be larger than adult-only groups. Neither direction of travel nor time of day were significantly related to group size. The population appears to contain a high percentage of cow–calf pairs, and this suggests that the study area may be used as a calving area during the summer.
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